Pressure retaining valve

ABSTRACT

A load-holding valve ( 1 ) having at least one main control piston ( 39 ) which is guided longitudinally displaceably in a single-piece or multiple-piece valve housing ( 29 ) and a non-return piston ( 43 ), wherein, in switching positions, in the form of a load-holding position a fluid-conducting connection between at least two connection points ( 37, 21 ) is shut off by means of the two pistons ( 39, 43 ), of which connection points ( 37, 21 ) one ( 37 ) serves to feed or discharge media and the other ( 21 ) leads to a hydraulic consumer, in the form of a load-raising position the two pistons ( 43 ) move relative to one another into a release position, in which the connection between the medium feed ( 37 ) and the consumer is established, and in the form of a load-reducing position the two pistons ( 39, 43 ) assume a predefinable spacing from one another in such a way that the connection between the medium discharge ( 37 ) and the consumer is established, is characterized in that there is a monitoring device ( 75 ) which determines the individual switching positions of said pistons ( 39, 43 ), in order to ensure reliable functioning of the load-holding valve ( 1 ).

The invention relates to a pressure retaining valve having at least one main control piston, which is guided longitudinally displaceably in a single-piece or multiple-piece valve housing and a non-return piston, wherein, in switching positions, in the form of

-   -   a load-holding position, a fluid-conducting connection between         at least two connection ports is shut off by means of the two         pistons, of which connection ports one serves to supply or         return media and the other leads to a hydraulic actuator     -   a load-lifting position, the two pistons move relative to one         another into a release position, in which the connection between         the media supply (37) and the consumer is established, and     -   a load-lowering position, the two pistons assume a predefinable         spacing from one another in such a way that the connection         between the media return and the consumer is established.

Pressure retaining valves of this type are prior art. In conjunction with hydraulic actuators in the form of load-lifting power cylinders, such valves also are referred to as counterbalance valves in technical terminology. Load-lifting power cylinders are frequently used in tools and equipment, also in the form of mobile devices such as excavators, forklifts and the like. Typically in such cases, an operator can control the pressure retaining valve by manual actuation of a control valve device. To prevent operating errors on the part of the staff in doing so, and preclude resulting hazards for both the equipment as well as for persons to the extent possible, due care shall be taken to ensure that the handling operations for the staff are simple and clear.

In view of these problems, the invention addresses the problem of providing a pressure retaining valve, for which the risk of incorrect operation and resulting malfunctions is minimized.

According to the invention, this problem is solved by a pressure retaining valve having the features of claim 1 in its entirety.

According to the characterizing portion of claim 1, one essential feature of the invention is that a monitoring device is provided, which detects the individual switching positions of the piston of the valve to ensure a reliable functioning of the pressure retaining valve. Due to the fact that the operator and/or an automatic machine control (PLC) has information about the respective switching state, the control operation proves to be simple and clear, substantially reducing the risk of incorrect operation.

With particular advantage, the monitoring device may be designed in such a way that the main control piston and/or the non-return piston interact with a signaling device, which inductively alters a field generated by a measuring device, as soon as the position of the signaling device changes in relation to the measuring device, due to the movement of the main piston and/or the non-return piston.

In a particularly advantageous manner, a kind of inductive travel measuring device is provided as a monitoring device, where the signaling device consists of a control unit connected to the main piston, the control unit having at least partially magnetic field altering constituents, in particular in the form of iron compounds, wherein the measuring device has at least one energizable winding coil, generating the magnetic field. Such an inductive measuring device can be integrated in the valve housing without difficulty or mounted as an additional housing to one end of the valve housing, from which the control unit extends into the additional housing.

In an advantageous manner, the control unit is part of a signal rod, the one free end of which engages with the main control piston and in the area of its other free end extends through a dual coil arrangement, whose individual coils are arranged in series at a predetermined spacing along the travel direction of the signal rod. In such an arrangement, the individual coils may be part of an inductive half-bridge, for a displacement measurement according to the so-called LVDT system (Linear Voltage Differential Transformer). Alternatively or additionally, the programming or design takes into account defined switching points, like a relaxation oscillator (Schmitt trigger). In that regard, only two switching points that can be easily monitored are then required. Another monitoring option is the use of commercially available Hall sensors.

The arrangement can advantageously be made in such a manner that a zero or starting position for the measuring device is calibrated relative to the valve housing which, when departed from in one direction or another due to the main control piston, starting from a predetermined output voltage, the respective switching position of the valve can be inferred in the manner of a switching point detection.

Preferably, the respective switching point determination for the unambiguous detection of the respective switching position is provided with a kind of hysteresis loop.

The measuring device can be particularly advantageously connected to evaluation electronics, preferably providing the operator with a visual indication of the preferred switching positions on an LED display. The electronic monitoring accomplishes an increase in the diagnostic coverage ratio (DC) in accordance with DIN EN ISO 13849. In doing so, a higher performance level (PL) can be achieved as well as a higher functional safety level.

In particularly advantageous exemplary embodiments, the valve housing has a stop shoulder for forming the zero or starting position, against which stop shoulder the non-return piston abuts in the hold-pressure position, wherein in the load lifting or load lowering position the main control piston moves forward or backward from a position close to the stop shoulder position towards a frontal housing port as the hydraulic actuator port.

In advantageous exemplary embodiments, in at least one position of the main control piston, a throttled fluid pressure, in addition to a main spring with a preferably adjustable spring force, acts on said main control piston, especially during lowering movements.

As a control valve means for controlling the pressure retaining valve for the media supply or return, said pressure retaining valve is advantageously connected to a control valve, in particular in the form of a 4/3-proportional directional valve. Pressure retaining valves should also include and comprise all types of counterbalance valves and all kinds of double check valves of the generic kind.

Below the invention is explained in detail using an exemplary embodiment shown in the drawing. In the drawings:

FIG. 1 shows a symbolic representation of the circuit of a hydraulic system for supplying an actuator in the form of a power cylinder having a pressure retaining valve according to an exemplary embodiment of the invention;

FIG. 2 a longitudinal section of the exemplary embodiment of the pressure retaining valve;

FIG. 3 shows an incomplete and, compared to FIG. 2, enlarged longitudinal section only of that part of the exemplary embodiment that adjoins a frontal housing port;

FIG. 4 shows a, compared to FIG. 3, further enlarged, partial longitudinal section of only that section of the exemplary embodiment that is closer to the frontal housing terminal, the switching position “hold pressure” being shown, and

FIGS. 5 and 6 show representations corresponding to FIG. 4, wherein the switching positions “lift load” or “lower load” are shown.

As shown in the circuit illustration of FIG. 1, the exemplary embodiment of the pressure retaining valve 1 according to the invention, described in detail below, is provided for controlling the operation of a double acting power cylinder 3. For controlling the pressure retaining valve 1, a control valve in the form of a proportional 4/3-directional valve 5 is provided. Its pressure port P and its tank port T are connected to a pressure pipe 7 or tank pipe 9 of a supply unit 11. The pressure pipe 7 connected at the pressure side of a motor-driven hydraulic pump 13 is protected towards the tank pipe 9 via a pressure relief valve 15.

The 4/3-directional valve 5 is spring biased to the position shown in FIG. 1, corresponding to the switch position “ hold pressure” of the pressure retaining valve 1. From this valve position, the 4/3-directional valve 5 can be moved by manually controlling its valve piston 17 to the right (with reference to FIG. 1) to the valve position corresponding to the switching position “lift load” of the pressure retaining valve 1, and by shifting to the left to the valve position, which corresponds to the switching position “lower load” of the pressure retaining valve 1.

One actuator port 21 of the pressure retaining valve 1 is connected to the rod side 23 of the power cylinder 3, whose the piston side 25 is connected to a connection port 27, which is located, as can be seen, in particular in FIGS. 3 to 6, on the valve housing 29 of the pressure retaining valve 1. The connection port 27 is also connected to a port 33 for media supply or media return at the directional valve 5, whose additional valve port 35 for the media supply or media return is connected to a housing port 37 of the pressure retaining valve 1. Its main piston 39 is biased into the switching position “hold pressure” shown in FIGS. 1, 3 and 4 by a spring force acting on it, of the adjustable main spring 41, wherein a non-return piston 43 is held by a non-return piston spring 45 in the direction of a locked position, in which the passage between the actuator port 21 and housing port 37 is blocked.

FIGS. 2 to 6 show details of the composition of the pressure retaining valve 1 having an axially movable main piston 39 and a non-return piston 43 inside. The main piston 39 is biased by the main spring 41 in the drawing to the right in the direction of the open end of the valve housing 29 forming the actuator port 21. The non-return piston 43 is biased by the non-return piston spring 45 for movement in the direction away from the actuator port 21 and abuts at the holding position (FIGS. 1, 3 and 4) and the lowering position (FIG. 6) a stop shoulder 47, which is formed at a housing insert 49. As can be seen most clearly in FIG. 4, the main piston 39 abuts the end of the non-return piston 43 at the holding position, after it has slightly overrun the stop shoulder 47. For these piston positions, the connection between the actuator port 21 and the housing port 37, which consists of two rows of holes in the longitudinal direction of the valve housing 29 consecutively arranged in the valve housing 29, is locked by the non-return piston 43, whereas the connection port 27, and thus the piston side 25 of the power cylinder 3, is connected to the tank port T via the valve port 33 (FIG. 1) of the directional valve 5.

For the transition from the holding position to the lifting position or the lowering position, the valve piston 17 of the 4/3-directional valve 5 is moved to the right, viewed in the transition to the lifting position in FIG. 1. The valve 5 thus connects the pressure port P to the housing port 37 of the pressure retaining valve 1, whereas the connection port 27, which is connected to the piston side 25 of the power cylinder 3, is released towards the tank port T. The pressure applied to the housing port 37 opens the connection to the actuator port 21 by moving the non-return piston 43 against the force of the spring 45, so that the load 51 is lifted by pressure buildup in the rod side 23 of the power cylinder 3. Simultaneously, a control pressure corresponding to the load pressure of the rod side 23 of the power cylinder 3 acts from the actuator port 21 side via a control pipe 53 on the main piston 39. Media flowing away from the side of the piston 25 during the lifting process reaches the tank port T via the connection port 27 and the valve port 33 of the directional valve 5.

If the directional control valve 5 is moved back from the lifting position to the holding position shown in FIG. 1, the actuator port 21 is blocked by the non-return piston 43 and the piston side 25 of the power cylinder 3 remains connected to the tank port T via the connection port 27 and the valve port 33 of the directional control valve 5.

If the valve piston 17 of the directional control valve 5 moves to the left, viewed from the lifting position in FIG. 1, for the switching position “lower load”, the pressure port P is connected to the piston side 25 of the power cylinder 3 via the valve port 33 and the connection port 27. For media return on the rod side 23 through the actuator port 21, a movement of the main piston 39 against the force of main spring 41 into a position shown in FIG. 6 is effected, because the media pressure supplied by the valve port 33 of the directional control valve 5 acts on the main piston 39 via the connection port 27 and via a restrictor 55 in an additional control pipe 57 (in FIG. 3 in the form of a control fluid chamber). In this position, the main piston 39, as shown in FIG. 6, forms a restriction with a variable control cross-section between its final rim 59 and an edge 61, the restriction being denoted by 63 in FIGS. 1 and 6 and controlling the media outflow to the housing port 37 from the actuator port 21 during lowering. As can be seen most clearly in FIG. 3, during lifting, a control pressure acts on the main piston 39 from the actuator port 21 via the control pipe 53, which acts on the main piston 39 in the same direction as the main spring 41. The control pipe 53 is formed by an axial bore in the main piston 39, which results in an annular section 65 formed by a piston shoulder. The pressure acting at the actuator port 21 also acts on the main piston 39 against the spring 41. The resultant acts as a force against the spring 41. Further, a pressure-limiting function is implemented, in which the valve 1 is set to the open position.

The pressure surfaces effective at the non-return piston 43 and at the main piston 39 are indicated in FIG. 3 by A1 through A4. During load lifting, the pump pressure at the housing port 37 opens the non-return piston in the form of a hollow piston 43 via the annular section A4-A1 against the non-return piston spring 45, which is clamped between the non-return piston 43 and a retaining ring 67 at the open end of the housing 29. During pressure holding, the load pressure at the actuator port 21 acts on the resulting surface A1-(A2-A3) against the biasing force of the main spring 41. During load lowering, the actuating pressure prevailing at the connection port 27 acts on the annular section A2-A1 and opens together with the load pressure at actuator port 21 the main piston 39 against the biasing force of the main spring 41.

Connected opposite the open end of the valve housing 29 forming the actuator port 21 is a spring housing 69 for the main spring 41. This is supported at one end via a spring plate 71 by the facing end of the main piston 39. The other end of the main spring 41 is supported on the end of a measurement housing 73 screwed into the spring housing 69, which, in turn, holds an end section of a monitoring device 75. This provides, for instance, a means for inductive position measurement by the known LVDT system (Linear Voltage Differential Transformer) and has two energizable, magnetic field-generating coil windings 77 and 79. As components of an inductive half-bridge, the winding coils 77, 79 act in conjunction with a signaling device in the form of a rod-like control unit 81, which is connected at one end to the main piston 39 and is axially movable with the latter, and at the other end has an end section 83 of a highly permeable ferrite that is movable as a coil core within the winding coils 77, 79.

Changes of the inductance of the coil windings 77, 79 caused by axial movements of the end section 83 corresponding to movements of the main piston 39 and the resulting changes in the bridge voltage Vout, the respective switching positions of the pressure retaining valve 1 are determined based on a zero or starting position, by means of evaluation electronics of the prior art. This starting position is, as schematically indicated in FIGS. 4 to 6 and referred to as the mechanical center, calibrated in such a way that it corresponds to the position of the final rim 59 of the main piston 39 in the switching position “hold pressure”. In case of a bridge circuit, this position can correspond to an output voltage “high”, on both outputs A1 and A2. During the transition to “lift load” the main piston 39 moves a distance S in FIG. 5 to the right, resulting in an output voltage.

For the unambiguous determination of a switching point, a hysteresis range 85 is taken into account, which extends over a small section of the distance. The same applies to the state shown in FIG. 6 “lower load” with a corresponding movement of the control piston 39 from the mechanical center towards the left. The corresponding characteristic output voltage in turn is used for the detection of the switching point in a hysteresis range 87 over a small distance section.

Altogether, for two switching channels the following states result according to the following table:

Channels A1 A2 Center or neutral position high high Lift low high Lower high low

As FIGS. 2 and 3 show, an air bleeder 89 is formed in the spring housing 69. The rod-like control unit 81 is guided on the end section 75 projecting into the monitoring device in a guide sleeve 91, which is held in the measuring housing 73 in the bore 93 facing the main spring 41. 

1. A pressure retaining valve (1) having at least one main control piston (39), which is guided longitudinally displaceably in a single-piece or multiple-piece valve housing (29) and a non-return piston(43), wherein, in switching positions, in the form of a pressure-holding position, a fluid-conducting connection between at least two actuator ports (37, 21) is shut off by means of the two pistons (39, 43), of which connection ports one serves to supply or return media and the other leads to a hydraulic actuator, a load-lifting position, the two pistons (35, 43) move relative to one another into a release position, in which the connection between the media supply (37) and the actuator (3) is established a load-lowering position, the two pistons (39, 43) assume a predefinable spacing from one another in such a way that the connection between the media return (37) and the actuator (3) is established, characterized in that there is a monitoring device (75), which determines the individual switching positions of the aforementioned pistons (39, 43) in order to ensure a reliable functioning of the pressure retaining valve (1).
 2. The pressure retaining valve according to claim 1, characterized in that the main control piston (39) and/or the non-return piston (43) interact with a signaling device (81, 83), which inductively alters a field generated by a measuring device (77, 79), as soon as the position of the signaling device (81, 83) changes in relation to the measuring device (77, 79), due to the movement of the main piston (39) and/or the non-return piston (43).
 3. The pressure retaining valve according to claim 1, characterized in that the signaling device consists of a control unit (81, 83) connected to the main piston (39), the control unit having at least partially magnetic field altering constituents (83), in particular in the form of iron compounds, and in that the measuring device has at least one energizable winding coil (77, 79), generating the magnetic field.
 4. The pressure retaining valve according to claim 1, characterized in that the control unit (83) is part of a signal rod (81), the one free end of which engages with the main control piston (39) and in the area of its other free end extends through a dual coil arrangement, whose individual coils (77, 79) are arranged in series at a predetermined spacing along the travel direction of the signal rod (81).
 5. The pressure retaining valve according to claim 1, characterized in that a zero or starting position for the measuring device (77, 79) is calibrated relative to the valve housing (29); which, when departed from in one direction or the other due to the main control piston, starting from a predetermined output voltage, allows an inference to be made about the respective switching position of the valve (1) in the manner of a switching point detection.
 6. The pressure retaining valve according to claim 1, characterized in that the respective switching point determination for the unambiguous detection of the respective switching position is provided with a kind of hysteresis loop (85, 87).
 7. The pressure retaining valve according to claim 1, characterized in that the measuring device (77, 79) is connected to evaluation electronics, preferably providing a visual indication of the preferred switching positions on an LED display.
 8. The pressure retaining valve according to claim 1, characterized in that the valve housing (29) has a stop shoulder (47) for forming the zero or starting position, abutted by the non-return piston (43) in the load holding position, wherein in the load lifting or load lowering position the main control piston (39) moves forward or backward from a position close to the shoulder stop (47) position towards a frontal housing connection as the hydraulic actuator port (21).
 9. The pressure retaining valve according to claim 1, characterized in that in at least one position of the main control piston (39), a throttled fluid pressure (57) acts on the main control piston (39), in addition to a main spring (41) having a preferably adjustable spring force, especially during lowering operations, as well as a control pressure (53), which is controlled by the housing port (21) of the hydraulic actuator (3).
 10. The pressure retaining valve according to claim 1, characterized in that for controlling the pressure retaining valve for the media supply or return, it is advantageously connected to a control valve, in particular in the form of a 4/3-proportional directional valve (5).
 11. The pressure retaining valve according to claim 1, characterized in that in at least one position of the main control piston, a throttled fluid pressure (57), in addition to a main spring (41) having a preferably adjustable spring force, acts on said hydraulic actuator (3, 23), in particular during lowering operations, as well as a control pressure (53) during lowering operations, which is controlled by the other actuator port (21) of the hydraulic actuator. 